Rabies is a viral infection of mammals, including man, usually spread through bites of infected carnivores (Order Carnivora) or bats (Order Chiroptera). Rabies virus, as used here, refers to all members of the worldwide viral genus Lyssavirus (Smith, Clinical Microbiology Reviews 9:166–176, 1996) and new members of that genus that continue to be discovered (e.g., Gould et al., Virus Research 54:165–187, 1998), whatever future taxonomic revisions may suggest. These viruses typically produce encephalitis in mammalian hosts, resulting in transmission of virus to subsequent victims via bites or contamination of mucosal surfaces (Constantine, Rabies. In Hoeprich et al., Infectious Diseases. Lippincott, 1994) and sometimes via aerosols (Constantine, USPHS Publ. 1617, 1967). The virus occurs nearly worldwide in wildlife and in domestic dogs and cats, particularly where pet vaccination is not practiced. Some 50,000 persons and millions of animals reportedly die of the disease each year, and 8 million persons take antirabies treatment annually.
Rabies control is expensive but has been partially achieved in developed countries by postexposure antirabies treatment of people and vaccination of pets. Local destruction of wild carnivore populations during outbreaks has had some temporary effect, but the practice has come under increasing opposition by conservationists and has been outlawed in many areas.
In recent years the vaccination of wild carnivore poulations has been attempted as a control measure, using live vaccines, but both target and nontarget species sometimes have been infected by the vaccines, producing clinical rabies. Safety measures have been lacking, and results are of questionable value.
The insect-eating bat populations of North America generally have been spared control by destruction, because they are needed to control insects, and only about one-tenth of one per cent are known to be infected with rabies.
It is apparent that the ideal approach to animal rabies control would be to eliminate only rabies-infected animals, a challenge that nobody has dared to consider. The inventor pondered this dilemma and concluded that success might result from exploiting differences between rabid and uninfected animals. A major difference is the insane, attack-prone behavior of the rabid animal. Moreover, knowledge of the disease in man and animals has demonstrated that the sensory systems (auditory, visual, tactile, odor, taste) of infected individuals develop an increased sensitivity and responsiveness to sensory stimuli, a condition known as hyperesthesia.
This increased sensitivity to stimuli has resulted in various abnormal responses, the most noticeable one being attacks by infected individuals on animate or inanimate sources of a sensory stimulus. For example, rabid animals are known to attack persons who are yelling, laughing, shooting a gun, or driving a vehicle, and moving or noisy animals or vehicles have been attacked. Therefore, it appeared promising that sensory stimuli might be used to induce rabid animals to attack the source and be trapped or destroyed in the process. But suitable stimuli should be specific for rabid animals and should not attract normal animals. Thus, odor and taste would not be promising candidates. Tactile stimili, such as touching or contrasting temperatures, probably could be used only secondary to a more specific primary stimulant, because they would not be specific enough in themselves. Air movement, a strong rabies behavior tactile stimulant, would seem useful, at least in a secondary role, but it might be attractive to some normal animals if used alone. Useful visual stimuli, such as flashing bright lights or moving objects, including motor vehicles, appeared either environmentally objectionable, impractical in most instances, or disturbing to people and normal animals.
Auditory stimuli, with or without secondary stimuli, seemed to be promising except for disturbance of man and normal animals, an objection that might be circumvented by employing muted sounds or sounds above the hearing range of man, birds, reptiles, and most mammals but within the range of most target species (i.e., bats and carnivores) and preferably repellant or unattractive to uninfected animals.
Normal wild animals can be attracted or repelled in various ways, including the use of sounds. Natural or contrived sounds of prey will attract carnivores. Natural or playback of recorded bat calls will attract bats. However, the objective was not to attract normal animals but to lure only attack-prone, rabies-infected animals. Some sounds (e.g., playback of recorded animal calls) would be expected to attract both rabid and normal animals if for different reasons. Such sounds could not be used, because they would lure normal animals.
Certain species of bats were chosen for initial studies to discern a pattern or model for subsequent application to other mammal groups, primarily carnivores. The study concentrated on the western pipistrelle bat (Pipistrellus hesperus), the more abundant of three species of bats then known to engage in rabies-driven attacks on people in the western United States (Constantine, Public Health Reports 82:867–888, 1967).
Of the sensory stimuli to be considered as initiators of attacks (auditory, visual, tactile, odor and taste), auditory stimuli appeared to be the most promising to use as a primary lure, because nonbiological sounds (neither actual, recorded or simulated animal sounds) should be attractive to rabid bats and/or rabid carnivores but not attractive or possibly repellent to normal, uninfected animals. Moreover, the hearing abilities of bats and wild carnivores extend into ultrasonic ranges, up to 200 kilohertz or kHz in some bats, well above the hearing abilities of man and domestic herbivores, 16–20 kHz and 22 kHz respectively (Fay and Popper. Comparative Hearing: Mammals. Springer-Verlag, 1994). In this respect ultrasound would appear ideal for this purpose, a view tempered by knowledge that ultrasound does not carry as far as audible sound; it dissipates quickly as distance from the source increases.
Attacks on persons by flying rabid bats occurred during daylight in incidents wherein the time was recorded. Of 37 attacks, sound associated with the victim was noted or implied in all cases, usually yelling due to fear of a flying bat, although some victims were driving noisy, open heavy vehicles, one was operating a jackhammer, and two were target shooting. Three barking dogs were attacked. Thus, sounds were consistently associated with attacks, but motion, often associated with sound, might be a factor.
The inventor surveyed large areas of California by aircraft, 4-wheel drive vehicles, and on foot, and he captured many thousands of bats to locate areas where maximal concentrations of these bats occur. He then proceeded with nighttime trials in efforts to attract attacking bats into traps, using a variety of sounds as potential lures. Sounds ranged from recorded noises associated with attacks (e.g., jackhammer, motorcycle) through nonspecific sounds (e.g., jet engines, smoke alarm, whistles) to a variety of ultrasounds. These nighttime trials were not successful.
Finally, he reasoned that his nighttime efforts were competing with myriads of normal bats that logically are targeted by rabid bats. Since rabid bats are also active in daytime, when normal bats rarely fly, trials should be done in daylight to escape the nighttime competition offered by normal bats. Therefore, a trap equipped with a smoke alarm lure was installed in a high bat density area south of Death Valley and kept active day and night. On the third day it contained an extremely belligerent rabid bat.
The smoke alarm lure has a sound intensity of 105 decibels at ten centimeters and a sound frequency range from five to 20 kHz, about 95 per cent of which is at the lower (audible) end of the range. It could be heard at a distance of a quarter mile. As the alarm sound can be a nuisance to people and normal animals, the alarm was replaced by another device of similar sound intensity, but at an inaudible frequency, 48 kHz. Though inaudible by man, domestic herbivores, and most wildlife and consequently detectable from a lesser distance than the successful smoke alarm, the device sufficed to attract many rabid bats, as described below.
The inventor distributed a series of bat traps bearing the aforementioned type of ultrasonic lure throughout the same and similar areas wherein a total of 66 rabid (only) bats have now been taken. The nonbiological sound proved most effective when emitted in short bursts, such as one-third of a second in duration, followed by equal intervals of silence. Adjacent control traps (traps that lacked sound lures) did not take any bats. Twenty three healthy (only) bats were taken simultaneously by the conventional mist netting method for comparative purposes.
The unexpectedly high level of local rabid bat trapping success provides assurance that this new method will be highly useful in severely reducing or eliminating rabies locally and in stopping the spread of rabies when strategically applied on a suitable scale.
In addition to the foregoing bat trapping activities, the inventor has used standard boxlike traps, designed to live trap carnivores, in combination with nonbiological sound lures to attract and capture rabid (only) carnivores. These trials, performed on skunks in the field as well as in the laboratory, have yielded only successful results. Adjacent traps the lacked sound attractants were used as controls. These trials with rabid carnivores are ongoing and will include additional species of wild as well as domestic carnivores to further confirm the technique's effectiveness for rabies control on a global level.